The invention relates to a field emission cathode device of an electrically conducting material and with a narrow, rod-shaped geometry or a knife edge to achieve high amplification of the electric field strength, such that the electron-emitting part of the field emission cathode has cylindrical molecules. The invention also relates to a method for producing such a field emission cathode device.
Field emission means the emission of electrons from the surface of an electric conductor under the action of an electric field exceeding 109 V/m. In practice, such field strengths are realized at sharp edges or tips, where the field strength is amplified. High vacuum is necessary to avoid gas discharges.
Field emission cathodes are used, for example, in field electron microscopes, in electron accelerators, in high-power switches (OS DE 39 24 745 A1) and in field emission diodes and field emitter arrays for vacuum microelectronics (thus for example Busta, Vacuum microelectronicsxe2x80x941992, Journal of Micromechanics and Microengineering, 2 (1992), pp. 53-60, and Iannazzo, A survey of the present status of vacuum microelectronics, Solid State Electronics, 36 (1993), pp. 301 to 320). A tungsten wire can be used as the field emission cathode, whose tip becomes so fine by etching that it can no longer be seen in an optical microscope. Also by etching, the ends of carbon fibers can be made sufficiently fine (Heinrich, Essig, Geiger, Appl. Phys. (1977) 12, pp. 197-202) to serve as a field emission cathode.
In vacuum microelectronics, field emission cathodes generally are produced by the methods of microprocess technology, by etching and sputtering, using lithographically produced masks (see Busta, Vacuum microelectronicsxe2x80x941992, Journal of Micromechanics and Microengineering, 2 (1992), pp. 53-60). By this method, one can produce conical tips with a radius of curvature of a few nm or wedge-shaped cutting edges with comparable radii of curvature. As materials for the cathode, one can use, for example, molybdenum, lanthanum hexaboride, hafnium, diamond-like carbon (B. C. Djubua, N. N. Chubun, Emission properties of Spindt-type cold cathodes with different emission cone material, IEEE Transactions on Electron Devices, 38 (1991) No. 10, pp. 2314-2316).
A disadvantage in the use of tips and edges, which have been produced by the known methods, is that the electron stream declines with operating time, since the tips or edges are destroyed by the positive ions of the unavoidable residual gas in the system. The like applies to field emission cathodes which are produced by sputtering techniques. The reason for this primarily is that the material structure of the emission tips is not uniquely defined. Thus, the geometry and microstructure of the tip and thus the work function of the electrons can vary within such wide limits that the electron streams from several tips, which were produced in one process, can differ by orders of magnitude, and furthermore change with operating time.
Furthermore, field emission cathodes for vacuum microelectronics cannot be produced in their optimal geometry by the prior art. Field strength calculations for various geometries of the tips show that the best shape of a field emission cathode is a narrow rod (Utsumi, Vacuum microelectronics: What""s new and exciting, IEEE Transactions on Electron Devices 38 (1991), pp. 2276-2283). The present methods of microstructure technology can produce at most wedge-shaped tips in a defined manner.
Carbon nano-cylinders were observed for the first time in an electron microscope by Iijima (Nature, 354 (1991), p. 56). They can now be produced in large quantities, for example at the cathode of a visible arc (Iijima, Materials Science and Engineering, B19 (1993), pp. 172-180). In the presence of iron or cobalt, one can produce single-shell carbon nano-cylinders. Theoretical calculations show that, depending on the helicity of the hexagonal ring structure, the walls of the carbon nano-cylinders are electrically conducting or semiconducting (Saito, Fujita, Dresselhaus, Dresselhaus, Materials Science and Engineering, B19 (1993), pp. 185-191). The carbon nano-cylinders can also be filled with metals, for example with lead. Other methods for producing carbon nano-cylinders are described in the literature:
Carbon nano-cylinders can be produced by the catalytic decomposition of acetylene through iron particles at about 700xc2x0 C. (Jose-Yacaman, Miki-Yoshida, Rendon, Applied Physics Letters 62 (6) 1993, pp. 657-659).
In the presence of methane, argon, and iron vapor, single-shell carbon nano-cylinders can be found in the carbon deposit on the chamber walls of a visible arc apparatus (Iijima, Nature 363 (1993), pp. 603-605).
By sputtering a carbon film in high vacuum, multiple-shell carbon nano-cylinders can be deposited on a graphite surface (Ge, Sattler, Science 260 (1993), pp. 515-518).
By sputtering ultra-pure graphite with electron beams in vacuum, carbon nano-cylinders can be produced on substrates consisting of various materials, such that the carbon nano-cylinders are aligned in the direction of the vapor jet (Kosakovskaya et al., JETP Lett., 56 (1992), p. 26).
In addition to the carbon nano-cylinders, disordered carbon particles generally are also deposited on the substrate. These can be removed, for example, by treatment in an oxidizing atmosphere at an elevated temperature up to 500xc2x0 C., preferably 400xc2x0 C. The carbon nano-cylinders at the end caps can be opened in a similar manner in an oxidizing atmosphere (air, CO2, or pure oxygen). This offers the possibility of filling the carbon nano-cylinders with metals, as described for a filling with lead by Ajayan and Iijima in Nature 361, p. 333.
In principle, it is possible to fasten these carbon nano-cylinders, produced by one of the above methods, on a suitable substrate, by means of micro-manipulators, and thus to produce a field emission cathode. However, this method is impractical, and in particular is not suited for field electron arrays with many cathode tips, as is desired in vacuum microelectronics.
Field emission cathodes with emission tips of bio-molecular microstructures or a metal-semiconductor-eutectic are known from the U.S. Pat No. 5,138,220. The diameter of these structures measures in the micrometer range, and subsequent metallization is necessary to achieve adequate emission.
The publication IBM Technical Disclosure Bulletin, Vol. 35, No. 7, December 1992, pp. 410-411 describes the use of Buckminster fullerene molecules as the tip of scanning-probe microscopes. Besides spherical C60 molecules, derivatives of C60 - and hetero-fullerenes are mentioned, that is host molecules in which individual C-atoms have been substituted by boron or nitrogen.
The present invention is based on the object or on the technical problem of specifying a field emission cathode which avoids the disadvantages of the prior art, assures high emission quality, makes possible a longer lifetime, and in particular resists bombardment with residual gas ions. Furthermore, the present invention is based on the object or on the technical problem of specifying a method for producing a field emission cathode of the type mentioned in the introduction, so as to assure technically optimal manufacture together with economy.
The inventive field emission cathode device consists of an electrically conducting material and having the shape of a narrow rod or a knife edge to achieve high magnification of the electric field strength, such that the electron emitting part of the field emission cathode has cylindrical molecules, wherein the cylindrical molecules are formed at least in part as single-shell or multiple-shell carbon nano-cylinders. The inventive method for producing the field emission cathode device with carbon non-cylinders which have been expanded during the gas phase. Advantageous modifications and developments are the subject of the subclaims.
An especially preferred design of the inventive field emission cathode is characterized in that carbon nano-cylinders are used as field emission cathodes. Single-shell carbon nano-cylinders with a diameter of about 1 nanometer and a length greater than 1 micrometer, or also multiple-shell ones with a diameter up to several nanometers can be produced. Bundles of single-shell carbon nano-cylinders with diameters of about 5 nanometers can also be produced. The walls of the carbon nano-cylinders consist of carbon atoms in a hexagonal pattern, while the end caps additionally contain 5-ring structures. The individual carbon atoms of the carbon nano-cylinders are strongly bound chemically, as a result of which the carbon nano-cylinders have extremely great mechanical strength. This also results in their high sputtering strength in comparison to randomly grown tips, which are sputtered according to the prior art.
Using well-known carbon nano-cylinders as a field emission cathode thus combines the advantage of optimal geometry with high strength, thus assuring that the emission properties of such field emission cathodes will not change during their operation, in contrast to previously used cathode tips.
So that the above advantages of the carbon nano-cylinders can also be used for making the field emission cathode arrays of vacuum microelectronics, the known methods for producing such arrays must be modified according to the invention, in such a way that the carbon nano-cylinders grow on appropriately prepared locations of a substrate.
The production method can be used to produce either individual field emission cathodes or also field emission cathode arrays.